10 year old causing issues between ex

[Strawber]

Me and DH are separated. Dc has a really bad habit of telling takes but to her dad and then to me. She also tells him if I even say one thing about him (even if it’s not mean). For example we were talking about name calling on the context of someone calling you crazy when they just don’t like what you’ve said to them’. She’s runs straight to y and tells him then me and ex end up fighting.

how to stop this

Don't discuss him with her at all, even if she asks you, other than about pickup arrangements etc. She may be trying to stir things up because it gives her a sense of power in a situation where she feels powerless. If she comes to you with "tales" about him, don't react, just ask her why she thinks you are interested. Explain that you are not together any more, and it will be easier for everyone if you can try to get on at a distance, but without arguing. Good luck.